The present invention relates to an insulated substrate having high thermal conductivity and a method of manufacturing the same.
The packaging density of semiconductor devices has become high with the reduction in size, increase in degree of integration and increase in output of electronic circuits due to the development in IC (Integrated Circuit), LSI (Large Scale Integration), and the like. As the degree of integration, output and density of semiconductor devices become high or large, the number of devices per one chip becomes large year by year, whereby the calorific value per one chip increases. There is strongly desired a material for a package having high thermal conductivity, because the increase of calorific value has a significant influence upon the reliability of semiconductor devices. Further, in the case of hybrid IC, heat-generating parts are disposed in the same package, so there is required an insulating substrate having high thermal conductivity in order to make the packaging density still higher. Still further, in consideration of the actual mounting of devices on the substrate, the substrate preferably should have a coefficient of thermal expansion similar to those of the devices, other material constituting the package and circuit substrates, and should allow chips to be bonded directly on the substrates.
As a substrate which satisfies the above requirements, there has been proposed a ceramic substrate made of Al+Al.sub.2 O.sub.3, crystalline SiC, BeO, AlN and the like; or an enamel substrate obtained by covering a metal substrate with ceramic. But the above substrates are all expensive except for the enamel substrate. The substrate made of BeO is toxic. The substrate made of crystalline SiC has a problem in that its dielectric constant at high frequency is large, i.e. it is above 40 at 1 MH, because BeO is used as an auxiliary for sintering in cystalline SiC. Further, the substrate made of Al+Al.sub.2 O.sub.3 has problems in that the thermal conductivity and heat resistance are insufficient, because the coefficient of linear expansion of the substrate is large and the insulating property of Al.sub.2 O.sub.3 is insufficient, thereby epoxy coating is required to compensate for these drawbacks. In the case of a cheap enamel substrate, it has problems in that its coefficient of linear expansion is large and thermal conductivity is poor. AlN has poor stability to water or alkali.
Recently, there have been produced carbon films having high hardness comprising diamond, diamond-like carbon, i-carbon and the like by means of an ion beam vacuum evaporation method, ion beam sputtering method, CVD method, plasma CVD method, and the like. The films have been taken notice of as hard insulating materials.
The term "diamond-like carbon" means a film wherein (1) diamond and amorphous carbon, (2) diamond and graphite, or (3) diamond, graphite and amorphous carbon are mixed.
In producing carbon films having high hardness by means of the plasma CVD method, and the like, carbon films having high hardness as deposited on a substrate by plasma decomposing a mixed gas of a hydrocarbon compound and H.sub.2, or an inert gas such as Ar or He.
The carbon film obtained by the above manner has large internal stress and selectivity to the substrate. That is, it has good adhesive property to a substrate comprising crystalline silicon, diamond, W, Mo, CuW and the like. However, it has large internal stress and small adhesive strength to a substrate comprising widely-used SUS (stainless steel), Al, Al alloy, Cu, Cu alloy, Ni, Zn, Zn alloy, whereby exfoliation takes place to shorten the life of the film. That is, in general, the carbon film is easy to exfoliate in the case of a substrate made of Al, Cu, Cu alloy, stainless steel, and the like having a large coefficient of thermal expansion. On the other hand, it is hard to exfoliate in the case of a substrate made of crystalline silicon, W, Mo, CuW and the like which have a coefficient of thermal expansion approximate to that of diamond.
Diamond has bee utilized as a material for a heat sink used in semiconductor devices for large power, microwave oscillating devices as VLSI, because diamond has high thermal conductivity and good electrical insulating property.
However, in producing a diamond-made thin film in practice, there are generally deposited amorphous carbon and graphite besides diamond when forming the film by means of vapor deposition, whereby the growth of diamond is disturbed. Further, the thus obtained thin film has a polycrystalline structure, and a graphite layer is formed at an intergranular portion thereof. Therefore, the diamond thin film produced in the above manner is very poor in electrical insulating property, particularly in dielectric breakdown.
The problems associated with a conventional thin film made of diamond and/or diamond-like carbon produced by several kinds of vapor phase growth methods are explained below.
(1) In the case of a heat filament CVD method, a tungsten heater for heating a substrate is heated up to about 2000.degree. C., accordingly a large amount of tungsten is vaporized, causing a problem in that the tungsten heater is exhausted in a short time and is easily cut. Further, there are generated graphite-like portions because of the nonuniformity in temperature caused by the change with the passage of time in the tungsten heater. Therefore, there is a problem in that it is difficult to form a film composed solely of diamond in producing a thin film having a large area.
(2) In the case of an ion beam sputtering method or ion plating method, the obtained thin film has problems in that the electrical resistivity of the film is lowered, causing its insulating property to become insufficient, because the above-mentioned amorphous carbon or graphite, except diamond, deposits in the thin film.
(3) In the case of a microwave CVD method, it is necessary, as described below, to add H.sub.2 to the reaction gas in order to generate hydrogen radicals to remove amorphous carbon or graphite generated in the thin film. That is, it is necessary to dilute CH.sub.4, which is a raw material gas, with H.sub.2 to lower the concentration of CH.sub.4 to not more than 1%, whereby the velocity of forming a thin film is limited. Further, in increasing the area of the film formed on the substrate, there is caused a problem in that graphite is formed due to nonuniformity of the concentration of hydrogen radicals.
When producing thin films made of diamond and/or diamond-like carbon in such manners as described above, it is necessary to etch amorphous carbon or graphite formed in the thin film with hydrogen radicals, and the like.
However, in practice, the concentration of hydrogen radicals, and the like is apt to become nonuniform throughout the surface of the substrate when, for instance, a thin film made of diamond and/or diamond-like carbon is formed on a silicon substrate. Therefore, amorphous carbon or graphite is locally formed and reduces the electrical insulating property. Every film obtained by the above methods has a polycrystalline structure. It is impossible to use substrates coated with these films as insulated substrates having high thermal conductivity, because the formation of graphite at intergranular portions is unavoidable and, therefore, sufficient electrical insulating property cannot be obtained. Further, at present, it is impossible to use the above films an electric insulating films, in case of producing films having a large area.
Further, there have been employed flame-coated substrates as metal base substrates, but these substrates have poor surface smoothness. It is impossible to form diamond, diamond-like carbon or SiC each having high thermal conductivity on a metal substrate by means of a flame coating technique at present.
It has been reported that diamond or diamond-like carbon could be formed on a crystalline silicon substrate, Mo substrate, W substrate or diamond substrate by means of an ion beam method, plasma CVD method, thermal CVD method, electron beam CVD method, and the like. But there has not yet been reported that diamond or diamond-like carbon could be formed at Al substrate, Al-Si substrate, Cu substrate, and Cu alloy substrate having high thermal conductivity, where are used widely and are cheap.
The present invention was made to solve the above problems and an object thereof is to provide an insulated substrate having high thermal conductivity and a method of manufacturing the same. That is, an object of the present invention is to provide an insulated substrate which does not have the drawbacks of the conventional substrates such as AlN, crystalline SiC and BeO, i.e. the substrate of the present invention has a coefficient of linear expansion closely akin to that of the material constituting packages such as devices, or circuit substrates; good thermal conductivity and heat resistance; smooth surface; and is able to produce circuits having thick films.
Another object of the present invention is to solve the above problem of exfoliation, to increase the adhesive strength, to reduce the internal stress and to increase the deposition velocity of heard carbon films.
A further object of the present invention is to provide an insulated substrate having high thermal conductivity comprising multilayer films, which has superior insulating property and high thermal conductivity.